In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user). In general, each coverage area may operate on one or more carriers each defining a respective downlink frequency range or “downlink channel” for carrying communications from the base station to UEs and a respective uplink frequency range or “uplink channel” for carrying communications from UEs to the base station. Further, both the downlink channel and uplink channel of each carrier may be divided into sub-channels for carrying particular communications, such as one or more control channels for carrying control signaling and one or more traffic channels for carrying application-layer data and other traffic.
In general, when a UE is positioned within coverage of a base station, the base station may serve the UE on a particular carrier and may allocate resources on that carrier for use to carry communications over the air to and from the UE.
For instance, in a system operating according to an orthogonal frequency division multiple access (OFDMA) protocol, such as the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS) for example, the air interface is divided over time into frames and sub-frames each defining two slots, and the uplink and downlink channels are divided over the bandwidth of the carrier into sub-carriers that are grouped within each slot into resource blocks. When a UE is positioned within coverage of a base station in such a system, the UE may register or “attach” with the base station, and the base station may then schedule particular downlink and uplink resource blocks on the air interface to carry data communications to and from the UE. Further, the base station and UE may modulate their air interface data communications at a coding rate selected based on quality of the UE's coverage, such as with higher rate coding rate when the UE is in better coverage of the base station and with a lower coding rate when the UE is in worse coverage of the base station.
In practice in such an LTE system, for instance, when the base station has data to transmit to a UE, the base station may assign certain downlink resource blocks (and thus certain sub-carriers) in a given sub-frame for use to carry the data to the UE at a particular coding rate, and the base station may then (i) transmit to the UE in that sub-frame a downlink control information (DCI) message that specifies the assigned resource blocks and coding rate and (ii) encode and transmit the data to the UE in the assigned resource blocks in that sub-frame. Per the DCI message, the UE would then read the transmitted data from the assigned resource blocks. Likewise, when the UE has data to transmit to the base station and accordingly transmits a scheduling request to the base station, the base station may assign certain uplink resource blocks in a given sub-frame for use to carry the data from the UE at a particular coding rate and may transmit to the UE, in advance of that sub-frame, a DCI message that specifies the assigned resource blocks and coding rate. The UE may then encode and transmit the data to the base station in the assigned resource blocks in that sub-frame.
With such an arrangement, the bandwidth of the carrier on which the base station serves a UE may define an effective limit on the rate of data communication between the base station and the UE, as the bandwidth would define only a limited number of resource blocks per slot, with data rate per resource block being further limited based on air interface conditions. By way of example, in accordance with the LTE standard, the uplink and downlink channels on each carrier may be 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, each resource block spans 180 kHz, and each slot is 0.5 milliseconds long. Accounting for guard bands at the edges of each carrier, the maximum number of resource blocks per 0.5 millisecond slot is thus 15 in 3 MHz, 25 in 5 MHz, 50 in 10 MHz, 75 in 15 MHz, and 100 in 20 MHz. Consequently, an LTE base station that serves UEs on such a carrier would have only the specified number of resource blocks available to allocate for air interface communication per slot, with coding rate in each resource block being further limited based on air interface conditions.
One way to help overcome this per-carrier data rate limitation is to have a base station serve a UE on multiple carriers at once, providing what is known as “carrier aggregation” service. With carrier aggregation service, multiple carriers from either contiguous frequency bands or non-contiguous frequency bands are aggregated together as “component carriers” to increase the overall bandwidth available per slot by providing a greater number of resource blocks in which the base station can schedule uplink and downlink communication. Further, where the concurrently-used component carriers are sufficiently distant from each other in the frequency spectrum, serving a UE concurrently on those component carriers may additionally create a frequency-diversity effect that could further improve data throughput.
To provide a UE with carrier aggregation service, a base station may engage in signaling with the UE to establish a radio-link-layer connection that encompasses multiple component carriers, and the base station may then schedule data communication to/from the UE concurrently on those multiple component carriers.
For instance, in an LTE system, a base station may engage in Radio Resource Control (RRC) signaling with the UE, transmitting to the UE an RRC configuration message that specifies the multiple component carriers on which the base station will serve the UE, so that the UE will then prepare itself (e.g., tune itself) to communicate on those carriers. When the base station has data to transmit to the UE in a given sub-frame, the base station may then assign downlink resource blocks including one or more resource blocks respectively in each of the multiple component carriers, and the base station may (i) transmit to the UE in the sub-frame a DCI message that designates the assigned resource blocks in the multiple component carriers and (ii) transmit the data to the UE in those assigned resource blocks. Likewise, when the UE has data to transmit to the base station, the base station may assign uplink resource blocks including one or more resource blocks respectively in each of the multiple component carriers, and the base station may (i) transmit to the UE a DCI message that designates the assigned resource blocks in the multiple component carriers and (ii) receive the data communication from the UE in the assigned resource blocks.